Gas discharge lamp is used to illuminate based on the theorem of gas discharge. The filaments of the gas discharge lamp are generally coated with emissive material. After the gas discharge lamp has been used for a long time, the emissive material coated on the filaments will be depleted. The loss of the emissive material will complicate the emission of the electrons by the filaments. When a considerable loss of the emissive material is happened, the lamp will enter the end-of-life (EOL) state.
Referring to FIG. 1 and FIG. 2, in which FIG. 1 shows the equivalent circuit of the gas discharge lamp and FIG. 2 shows the waveform of the lamp voltage of the gas discharge lamp. As shown in FIG. 1, the positive equivalent resistor Rp1 of the gas discharge lamp 1 and the negative equivalent resistor Rp2 of the gas discharge lamp 1 will have the same resistance, and their resistance will be varied along with the state of the gas discharge lamp 1. When the gas discharge lamp 1 is ignited, the resistance of the positive equivalent resistor Rp1 and the negative equivalent resistor Rp2 will change from a high resistance value to a low resistance value. Under this condition, the waveform of lamp voltage across the gas discharge lamp 1 in the positive half-cycle and the waveform of the lamp voltage across the gas discharge lamp 1 in the negative half-cycle are symmetrical, as indicated by the symbol Vpa of FIG. 2. When the gas discharge lamp 1 has been used for a long time and starts aging, i.e. when the lifetime of the gas discharge lamp 1 is going to an end, the discharging characteristics of the gas discharge lamp 1 in the positive half-cycle will be different from the discharging characteristics of the gas discharge lamp 1 in the negative half-cycle. That is, the resistance of the positive equivalent resistor Rp1 will be different from the resistance of the negative equivalent resistor Rp2. This would make the operation of the gas discharge lamp 1 abnormal. Under this condition, the waveform of lamp voltage across the gas discharge lamp 1 in the positive half-cycle and the waveform of the lamp voltage across the gas discharge lamp 1 in the negative half-cycle are asymmetrical, as indicated by the symbol Vpb of FIG. 2. Thus, the temperature of the cathode of the gas discharge lamp 1 will increase, which will damage the fixture socket of the gas discharge lamp 1, or break the glass of the gas discharge lamp 1.
Hence, it is intended to develop an end-of-life detector for gas discharge lamp and the ballast incorporating the same to precisely detect and determine if the gas discharge lamp has entered the end-of-life state, thereby providing necessary protection function.